1. Field of Invention
The present invention relates to a hydraulic pressure supply system of an automatic transmission for a vehicle. More particularly, the present invention relates to a hydraulic pressure supply system of an automatic transmission for a vehicle which can prevent unnecessary pressure override of a high pressure portion.
2. Description of Related Art
Recently, vehicle makers direct all their strength to improving fuel economy due to worldwide high oil prices and strengthening of exhaust gas regulations. Improvement of fuel economy may be achieved by improving power delivery efficiency, and the improvement of the power delivery efficiency may be achieved by minimizing unnecessary power consumption of a hydraulic pump.
A recent automatic transmission is provided with a low-pressure hydraulic pump and a high-pressure hydraulic pump so as to improve fuel economy. Therefore, hydraulic pressure generated by the low-pressure hydraulic pump is supplied to a low pressure portion (i.e., a torque converter, a cooling device, and a lubrication device), and hydraulic pressure generated by the high-pressure hydraulic pump is supplied to a high pressure portion (i.e., friction members selectively operated when shifting).
In further detail, general hydraulic pressure of the automatic transmission is generated for the low pressure portion (i.e., generated by the low-pressure hydraulic pump), and hydraulic pressure demanded by the high pressure portion is generated by the high-pressure hydraulic pump and then is supplied to the high pressure portion.
That is, the low hydraulic pressure generated by the low-pressure hydraulic pump is controlled by the low-pressure regulator valve and is supplied to the low pressure portion such as the torque converter, the cooling device, and the lubrication device, and the high hydraulic pressure generated by the high-pressure hydraulic pump is controlled by the high-pressure regulator valve and is supplied to the high pressure portion for operating the friction members related to shifting.
However, because the hydraulic pressure supplied to the high pressure portion is controlled only by the high-pressure regulator valve, unnecessary pressure override may occur according to a conventional hydraulic pressure supply system.
Particularly, if exhaust characteristics of an orifice disposed on a recirculation line of the high-pressure regulator valve is deteriorated at a low temperature or a target pressure of the high pressure portion is lowered and exhaust amount increases temporarily, pressure override may occur at the high pressure portion during delay in speed reduction of a motor. If pressure override occurs at the high pressure portion, friction members provided at the high pressure portion may be out of order and durability of the high pressure portion may be deteriorated.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.